The present invention relates to fine, electrically conductive lines, grids or circuits embedded in a substrate and a method for making such devices. In one embodiment the conductive grid is sufficiently fine to be invisible to the unaided eye and the substrate is a transparent thermoplastic sheet. Transparent conductive sheets have a variety of uses including resistively heated windows, electromagnetic interference (EMI) shielding, static shielding, antennas, touch screens for computer displays, and surface electrodes for electrochromic windows and liquid crystal displays.
The use of essentially transparent electrically conductive grids for such applications as EMI shielding is well known. The grid can be formed from a network or screen of metal wires that are sandwiched or laminated between transparent sheets or embedded in substrates (U.S. Pat. No. 3,952,152; U.S. Pat. No. 4,179,797; U.S. Pat. No. 4,321,296; U.S. Pat. No. 4,381,421; U.S. Pat. No. 4,412,255). The disadvantage of using wire screens is the difficulty in handling very fine wires or in making and handling very fine wire screens. For example, a 20 micron diameter copper wire has a tensile strength of only 1 oz (28 grams force) and is therefore easily damaged. Wire screens fabricated with wires of 20 micron diameter are available but are very expensive due to the difficulty in handling very fine wire.
Rather than embed a preexisting wire screen into a substrate, a conductive pattern can be fabricated in situ by first forming a pattern of grooves or channels in a substrate and then filling the channels with a conductive material. This method has been used for making conductive circuit lines and patterns by a variety of means, although usually for lines and patterns on a relatively coarse scale. The grooves can be formed in the substrate by molding, embossing, or by lithographic techniques and then filling the grooves with conductive inks or epoxies (U.S. Pat. No. 5,462,624), with evaporated, sputtered, or plated metal (U.S. Pat. No. 3,891,514; U.S. Pat. No. 4,510,347; U.S. Pat. No. 5,595,943), with molten metal (U.S. Pat. No. 4,748,130), or with metal powder (U.S. Pat. No. 2,963,748; U.S. Pat. No. 3,075,280; U.S. Pat. No. 3,800,020; U.S. Pat. No. 4,614,837; U.S. Pat. No. 5,061,438; U.S. Pat. No. 5,094,811).
These prior art methods have significant limitations, however. For example, one problem with conductive inks or epoxies is that the electrical conductivity is dependent on the formation of contacts between adjacent conductive particles, and the overall conductivity is usually much less than that of solid metal. Vapor deposition of metal or electroplating is generally fairly slow and often requires a subsequent step to remove excess metal that is deposited between the grooves. Molten metal can be placed in the grooves but usually first requires the deposition of some material in the grooves that the metal will wet. Otherwise the molten metal will not penetrate nor stay in the grooves due to surface tension of the molten metal.
Circuits have been made by placing metal powder into grooves followed by compacting the powder to enhance electrical contact between the particles. Lillie et al. (U.S. Pat. No. 5,061,438) and Kane et al. (U.S. Pat. No. 5,094,811) have used this method to form printed circuit boards. However, the method as described by these inventors is not practical for making very fine circuits and metal patterns. The described method forms a pattern of channels in a substrate by embossing the substrate with a patterned tool, places metal powder in the channels, and then uses the same tool to compact the powder. On a fine scale, replacing or re-registering the tool over the embossed pattern to perform the metal compaction would be extremely difficult. For example, a sheet with a pattern of 20 micron wide channels would require that the tool be placed over the pattern to an accuracy of roughly 3 microns from one side of the sheet to the other. For many applications, the sheet may be on the order of 30 cm by 30 cm. Dimensional changes due to thermal contraction of a thermoplastic sheet are typically about 1% or more during cooling from the forming temperature to room temperature. Thus, for a 30 cm by 30 cm sheet, a contraction of 1% would give an overall shrinkage of 0.3 cm. This value is 1000 times larger than the 3 micron placement accuracy needed, making accurate repositioning of the tool impossible.
Alternatively, Lillie et al. (WO85/01231) have suggested placing a deformable material such as plastic over the powder and then applying pressure to the deformable material to compact the metal powder into the grooves. Jack et al. (U.S. Pat. No. 3,075,280) apply pressure to an elastomer sheet over a pattern of particle filled grooves. In these cases, a relatively high pressure must be exerted to the back side of the plastic or elastomeric layer to create sufficient hydrostatic pressure in the region of a groove to cause the compliant layer to press into the groove to compact the powder. Grooves that are deep relative to their width would particularly pose a problem. If the compliant layer is a soft plastic it will engulf the particles making it impossible to remove the compliant layer without removing at least some of the particles.